Micromechanical component

ABSTRACT

A micromechanical component is proposed having a substrate ( 10 ) and a cover layer ( 40 ) deposited on the substrate ( 10 ), underneath the cover layer ( 40 ), a region ( 30; 30 ′) of porous material being provided which mechanically supports and thermally insulates the cover layer ( 40 ). On the cover layer ( 40 ), a heating device ( 70 ) is provided to heat the cover layer ( 40 ) above the region ( 30; 30 ′); and above the region ( 30; 30 ′), a detector ( 200, 200 ′) is provided to measure an electric property of a heated medium ( 150 ) provided above the region ( 30; 30 ′) on the cover layer ( 40 ).

FIELD OF THE INVENTION

[0001] The present invention is directed to a micromechanical componenthaving a substrate and a cover layer deposited on the substrate,underneath the cover layer, a region of porous material being providedwhich mechanically supports and thermally insulates the cover layer.

BACKGROUND INFORMATION

[0002] Although applicable to any number of micromechanical componentsand structures, particularly sensors and actuators, the presentinvention, as well as its basic underlying problem definition areexplained with reference to a micromechanical air-quality sensor whichcan be manufactured using the technology of silicon surfacemicromechanics.

[0003] Existing air-quality sensors are implemented using agas-sensitive material on a ceramic material. The gas-sensitive materialchanges its resistance and/or its dielectric properties in dependenceupon the concentration of the gas to be detected. To obtain a goodsensitivity, it is necessary to heat the gas-sensitive material. Thisdisadvantageously entails the use of a ceramic material and theassociated large type of design with respect to the substantial heatingpower to be expended and the long response time.

[0004] The method of etching silicon to make it porous (“anodizing”)constitutes related art, and it is described in numerous publications.The method of producing a cavity under a porous silicon layer islikewise already published (G. Lammel, P. Renaud, “Free-Standing Mobile3D Microstructures of Porous Silicon”, Proceedings of the 13^(th)European Conference on Solid-State Transducers, Eurosensors XIII, TheHague, 1999, 535-536).

SUMMARY OF THE INVENTION

[0005] An advantage of the micromechanical component according to thepresent invention is that it renders possible a simple andcost-effective manufacturing of a component having a thermallydecoupled, heatable cover-layer area, upon which a detector is provided.

[0006] For example, the use of porous silicon makes it relatively simpleto produce a deep cavity having a superjacent cover layer. Moreover, itis possible to make a defined region on a wafer porous up to a definedthickness, and, optionally, to oxidize to a higher valency in order tocreate a stable framework having low thermal conductivity.

[0007] In the exemplary implementation of an air-quality sensor usingthis method, one obtains the following further advantages:

[0008] low power consumption due to good thermal decoupling;

[0009] integration of a sensor element on the chip;

[0010] possible integration of a circuit on the sensor element;

[0011] very small size, along with any desired geometry of the porousregion;

[0012] low response time because of the small mass that has to beretempered;

[0013] capacitive or resistive evaluation possible;

[0014] different materials are usable for the heating and/or measuringresistors or electrodes;

[0015] a plurality of gas-sensitive materials may be employed on onechip.

[0016] An idea underlying the present invention is to provide, on thecover layer, a heating device to heat the cover layer above the region;and to provide, above the region, a detector to measure an electricproperty of a heated medium provided above the region on the coverlayer.

[0017] In accordance with one preferred further refinement, the porousmaterial is formed from the substrate material. This is readilypossible, particularly in the case of a silicon substrate.

[0018] In accordance with another preferred refinement, a hollow spaceis formed underneath the region of porous material.

[0019] In accordance with yet another preferred refinement, the coverlayer is formed by oxidizing the substrate surface and the surface ofthe porous region. This eliminates the need for depositing an additionalcover layer.

[0020] In accordance with yet another preferred refinement, the regionof porous material is completely oxidized. An oxidation of this kind isreadily possible because of the porous structure, and it enhances thethermal insulating capability.

[0021] Yet another preferred refinement provides for the component to bean air-quality sensor, the medium being a gas-sensitive medium, and thedetector having a capacitance detector and/or a resistance detector.

[0022] Still another preferred refinement provides for the detector tohave printed conductors arranged on the cover layer.

[0023] Yet another preferred refinement provides for the detector tohave printed conductors arranged on the insulation layer.

[0024] In yet another preferred refinement, the heating device extendsat least partially underneath the medium.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 shows a plan view of an air-quality sensor in accordancewith a first specific embodiment of the present invention.

[0026] FIGS. 2-4 show manufacturing steps for manufacturing theair-quality sensor according to FIG. 1.

[0027] FIGS. 5-6 show manufacturing steps for manufacturing anair-quality sensor in accordance with a second specific embodiment ofthe present invention.

[0028]FIG. 7 shows a cross-sectional view of an air-quality sensor inaccordance with a third specific embodiment of the present invention.

[0029] FIGS. 8-9 show manufacturing steps for manufacturing anair-quality sensor in accordance with a fourth specific embodiment ofthe present invention.

DETAILED DESCRIPTION

[0030] In the figures, components which are the same or functionallyequivalent are denoted by the same reference numerals.

[0031]FIG. 1 is a plan view of an air-quality sensor in accordance witha first specific embodiment of the present invention.

[0032] In FIG. 1, reference numeral 6 denotes contact surfaces orcontact pads; 10 a semiconductor substrate; 40 a cover layer situated onthe surface of semiconductor substrate 10; and 300 the boundary of aregion in which, underneath cover layer 40, a region 30 (compare, e.g.,FIG. 3) of porous material is provided which mechanically supports andthermally insulates cover layer 40. In the present case, the substratematerial is silicon and the porous material is anodized (porouslyetched) silicon.

[0033] In addition, reference numeral 50 denotes an insulation layerprovided above cover layer 40; 70 a heating resistor between cover layer40 and insulation layer 50; 350 the boundary of a region in whichinsulation layer 50 is removed from above cover layer 40; 200 aninterdigital capacitor situated on cover layer 40; and 150 denotes agas-sensitive material which covers the interdigital capacitors.

[0034] To operate the sensor structure shown in FIG. 1, gas-sensitivematerial 150 is heated by heating resistors 70, and the capacitance ofinterdigital capacitors 200 is measured in a generally known manner. Thegas-sensitive material changes its dielectric properties in dependenceupon the concentration of the gas to be detected. In this manner, thegas quality or concentration is able to be determined.

[0035] FIGS. 2-4 show manufacturing steps for manufacturing theair-quality sensor in accordance with FIG. 1.

[0036] In FIG. 2, in addition to the reference numerals alreadyintroduced, 15 denotes a mask, such as a resist mask, and 100 denotescircuit components of a sensor circuit that is not explained moreclosely. Substrate 10 shown in FIG. 2 is a silicon substrate.

[0037] According to FIG. 3, using the known method of porous etching, astructure is produced in which the substrate material is made porous ina certain region 30, and a hollow space 20 is subsequently formedunderneath porous region 30. Thus a part of porous region 30 is removed,so the result is the structure shown in FIG. 3.

[0038] To produce the structure shown in FIG. 4, following removal ofmask 15, porous region 30 is sealed by depositing cover layer 40, made,for example, of nitride, oxide, oxinitride, silicon carbide, orpolysilicon. Another possibility for forming cover layer 40 provides foroxidizing the substrate surface and the surface of porous region 30.

[0039] It is not essential for this airtight sealing of hollow space 20to follow the fabrication of hollow space 20, rather, it may also beaccomplished as one of the last process steps. The latter has theadvantage that, during processing, cover layer 40 does not bump out,which would lead to aberrations in a structuring process. The internalpressure that ultimately arises in hollow space 20 is dependent upon thepressure conditions prevailing during deposition or oxidation.

[0040] The measuring capacitors of interdigital capacitor 200, heatingresistors 70, and optional measuring resistors (not shown) are thenproduced on cover layer 40. Further functional layers may be depositedand patterned between cover layer 40 and the printed conductors ofheating resistors 70, i.e., above the printed conductors.

[0041] Above the measuring capacitors of interdigital capacitor 200,following application of insulation layer 50 which protects the formedstructure from environmental influences, gas-sensitive material 150 isapplied, which changes its dielectric properties as a function of theconcentration of a gas to be recorded.

[0042] The specific embodiment at hand has a hollow space 20, having anenclosed vacuum underneath cover layer 40, and region 30, in order toensure a good thermal insulation with respect to substrate 10 whengas-sensitive material 150 is heated by heating resistors 70.

[0043] FIGS. 5-6 illustrate the manufacturing steps used to manufacturethe air-quality sensor in accordance with a second specific embodimentof the present invention.

[0044] In the second specific embodiment shown with reference to FIGS. 5and 6, no hollow space is formed underneath substrate region 30′ thathas been made porous. Rather, following removal of mask 15, porousregion 30′ is immediately sealed by deposition of cover layer 40 or bythe oxidation.

[0045] In this context, the oxidation (not shown) has the advantage thatthe oxide has a lower thermal conductivity than the silicon, making itpossible to ensure a better decoupling from substrate 10. As in thefirst specific embodiment, the printed conductors, etc., are produced oncover layer 40.

[0046]FIG. 7 is a cross-sectional view of an air-quality sensor inaccordance with a third specific embodiment of the present invention.

[0047] In the third specific embodiment shown in FIG. 7, heatingresistors 70 are provided on cover layer 40, and the measuringcapacitors of interdigital capacitors 200′ are provided on insulationlayer 50, thus not directly on cover layer 40 as in the above exemplaryembodiments. The advantage of this arrangement is that the heatingstructure may be placed directly underneath gas-sensitive material 150.

[0048] FIGS. 8-9 depict manufacturing steps for manufacturing anair-quality sensor in accordance with a fourth specific embodiment ofthe present invention.

[0049] In accordance with FIG. 8, a two-layer substrate 10′, 10″ isprovided, in which an epitaxial layer 10″ is provided on a wafersubstrate 10′. Evaluation circuit 100 is additionally insulated by aburied region 110. The benefit of such a design is that the formation ofporous region 30, 30′ on bottom wafer substrate 10′ may be stopped byproperly doping components 10′, 10″.

[0050] Although the present invention is described above on the basis ofpreferred exemplary embodiments, it is not limited to them, and may bemodified in numerous ways.

[0051] In the above examples, the air-quality sensor according to thepresent invention has been presented in simple forms in order toelucidate its basic principles. Combinations of the examples andsubstantially more complicated refinements using the same basicprinciples are, of course, conceivable.

[0052] For example, instead of changing the dielectric properties, it isalso possible to change the electric resistance of the medium, e.g., ofthe gas-sensitive medium, using appropriate measuring electrodes.

[0053] In addition, it is possible to selectively etch porous region 30,30′ subsequently to or in-between the above process steps. For thispurpose, one or a plurality of openings may be produced in cover layer40, through which a selectively acting etching medium, in a fluid orgaseous state, is able to partially or completely dissolve out theporous region. The openings may subsequently be sealed again, a vacuumbeing preferably enclosed in hollow space 20 in the process in order toensure an optimal thermal decoupling between cover layer 40 andsubstrate 10. The openings may likewise be deliberately not closed. Inthis manner, the middle cover layer region having functional elementsmay be formed in such a way that it is only still joined by a few landfeatures (resist lines) to the substrate outside of the cavity (e.g.,connection by only two land features in the form of a bridge).

[0054] Also possible is the additional integration of a temperaturesensor on the cover layer outside of the porous region in order toprecisely set or regulate the desired temperature.

[0055] It is also possible to provide different media on the cover layeror the insulation layer above the porous region which are sensitive tovarious gases. This makes it possible to measure a plurality of gasesusing the same sensor element.

[0056] In addition, it is possible to realize the porous region so thatit continues right through to the bottom side of the substrate.

[0057] Finally, any micromechanical base materials may be used, and notonly the silicon substrate cited exemplarily.

[0058] In addition, the electric leads (not shown in FIG. 7) to theinterdigital structures may be situated underneath an electricallyinsulating protective layer. Also, the electrical connection by contactvias (openings) in the insulation layer may be implemented by electricalleads which are situated in the same plane as heating resistors 70. 10;10′, 10″ Si substrate  6 contact pads  40 cover layer 300 boundary ofporous region under 40 350 boundary region without insulation layer  70heating resistor 200, 200′ interdigital capacitor 150 gas-sensitivemedium  15 mask 100 evaluation circuit 110 buried layer  20 hollow space30, 30′ porous region  50 insulation layer

What is claimed is:
 1. A micromechanical component comprising: asubstrate (10), and a cover layer (40) deposited on the substrate (10),underneath the cover layer (40), a region (30; 30′) of porous materialbeing provided, which mechanically supports and thermally insulates thecover layer (40), wherein on the cover layer (40), a heating device (70)being provided to heat the cover layer (40) above the region (30; 30′);and above the region (30; 30′), a detector (200, 200′) being provided tomeasure an electric property of a heated medium (150) provided above theregion (30; 30′) on the cover layer (40).
 2. The micromechanicalcomponent as recited in claim 1, wherein the porous material of theregion (30; 30′) is formed from the substrate material.
 3. Themicromechanical component as recited in claim 1 or 2, wherein,underneath the region (30) of porous material, a hollow space (20) isformed.
 4. The micromechanical component as recited in one of thepreceding claims, wherein the cover layer (40) is formed by oxidizingthe substrate surface and the surface of the porous region (30; 30′). 5.The micromechanical component as recited in one of the preceding claims,wherein the region (30; 30′) of porous material is completely oxidized.6. The micromechanical component as recited in one of the precedingclaims, wherein the component is an air-quality sensor, the medium (150)is a gas-sensitive medium, and the detector (200, 200′) has acapacitance detector and/or a resistance detector.
 7. Themicromechanical component as recited in one of the preceding claims,wherein the detector (200, 200′) has printed conductors (200) situatedon the cover layer (40).
 8. The micromechanical component as recited inone of the preceding claims 1 through 6, wherein the detector (200,200′) has printed conductors (200′) situated on the insulation layer(50).
 9. The micromechanical component as recited in one of thepreceding claims, wherein the heating device (80) extends at leastpartially underneath the medium (150).